Why does the Torah see fit to add "Chamoro" and "Simlaso"?
Ramban: Even though a donkey is a non-Kasher animal and a garment not so precious to the owner like an animal, and although it is not so commonly lost.
Why does the Torah add "ve'Chein Ta'aseh le'Chol Aveidas Achicha"?
Rashi (on the Beraisa of Rebbi Yishmael): This is an example of a K'lal ('Shor, Seh, Chamor and Simlah') u'Perat ("Kol Aveidas Achicha"), which includes whatever is similar to any of the four specified articles.
Ramban: One is even Chayav to return household articles that are not as valuable as a garment that one wears.